This kind of control has conventionally employed a steering control system capable of controlling forward, rearward, rightward and leftward operations, particularly to be controlled by a single control lever by only a wrist work of the left hand with the left elbow being kept on a left side arm rest. In this case, a construction vehicle moves forwardly when the control lever is shifted from the neutral position to the forward position and moves rearwardly when the lever is shifted to the rearward position, and is steered to the left when the lever is shifted to the left and to the right when the lever is shifted to the right. Though there is no problem in control operations for forward and rearward movement, a mechanical linkage type steering control system comprising a plurality of control levers and rods in combination as shown in FIG. 3 has been employed for transmitting operating forces of these control levers and rods to actuate the clutch and the brake. FIG. 3 shows an example of the steering control system provided with two exclusively used control levers rather than with a single control lever. In this case, a rod type control lever 21 is connected to one end of a lever 23 having a pivotal point 22 and one end of a rod 24 is connected to the other end of the lever 23. The other end of rod 24 is coupled to a brake lever 25 and, when the brake lever 25 is actuated around the pivotal point 26 as a center, a brake 28 is ON and OFF-operated through a lever 27. A loose spring 29 is coupled to this brake lever 25 and the other end of this loose spring 29 is coupled to a steering control clutch valve 31 through a lever 30. When this steering control clutch valve 31 is actuated, a hydraulic pressure is applied to a clutch piston 32 through a piping 33 and the clutch 34 is disengaged. In the drawing, 35 and 36 denote gears and 37 denotes a spring.
However, this mechanical linkage type steering control system is disadvantageous in that the construction is complicated, transmission efficiency is low, operation strokes of control means are long and operating forces required are heavy. This type of steering control system is accompanied by the problem that, when the brake is worn out, the operation strokes vary and a force for tightening the brake band does not respond to control operation.